Aromatic polyesters typified by polyethylene terephthalate and polyethylene-2,6-naphthalene dicarboxylate are widely used in films because they have excellent mechanical properties, dimensional stability and heat resistance. Especially polyethylene-2,6-naphthalene dicarboxylate has more excellent mechanical properties, dimensional stability and heat resistance than polyethylene terephthalate and are used in fields in which these requirements are severe, for example, base films for high-density magnetic recording media. However, the requirement for dimensional stability in the high-density magnetic recording media is becoming higher and higher and the further improvement of the characteristic properties is desired.
Meanwhile, Patent Documents 1 to 4 propose polyesters comprising an ester unit of 6,6′-(ethylenedioxy)di-2-naphthoic acid as the main acid component and a diol component. These documents disclose crystalline polyesters having a melting point of 294° C.
However, the polyesters disclosed by these documents have a very high melting point and very high crystallinity. Therefore, when these polyesters are to be molded into a film, as they have low fluidity in a molten state, extrusion may become nonuniform, or when they are to be stretched after they are extruded, crystallization proceeds, whereby their films may be broken when they are stretched at a high draw ratio.
Patent Document 3 discloses a flexible disk made of a polyester comprising an ester unit of 6,6′-(ethylenedioxy)di-2-naphthoic acid as the main acid component and a diol component. It is disclosed that this flexible disk has a maximum temperature expansion coefficient (αt) of 10 to 35 (×10−6/° C.), a maximum humidity expansion coefficient (αh) of 0 to 8.0 (×10−6/% RH), a difference between the maximum and minimum temperature expansion coefficients (αt) of 0 to 6.0 (×10−6/° C.) and a difference between the maximum and minimum humidity expansion coefficients (αh) of 0 to 4.0 (×10−6/% RH).
In an Example of the above document, there is disclosed a film having a maximum temperature expansion coefficient (αt) of 19 (×10−6/° C.), a minimum temperature expansion coefficient (αt) of 16.5 (×10−6/° C.), a maximum humidity expansion coefficient (αh) of 6 (×10−6/% RH), and a minimum humidity expansion coefficient (αh) of 4.5 (×10−6/% RH).
However, the requirement for the improvement of recording density in magnetic recording media is now severe, and dimensional stability required for a base film cannot be attained not only by polyethylene terephthalate but also by polyethylene-2,6-naphthalene dicarboxylate and the film proposed by Patent Document 3.    (Patent Document 1) JP-A 60-135428    (Patent Document 2) JP-A 60-221420    (Patent Document 3) JP-A 61-145724    (Patent Document 4) JP-A 6-145323